Inorganic electroluminescent devices such as LED's are known in the art. The device generally comprises a chip of phosphor disposed between two electrodes. The phospher generally comprised a rectifying semiconductor such as gallium arsenide doped with aluminum or phosphorous. Upon application of a current, the gallium arsenide emits light. However, due to the complexity of manufacture, these devices can not be easily fabricated in large area displays or arrays.
Organic electroluminescent devices (materials and structure) are also known in the art as disclosed in Tang, U.S. Pat. No. 4,356,429 Shi et al., U.S. Pat. No. 5,593,788 and Heeger, U.S. Pat. No. 5,408,109, the disclosures of which are incorporated herein by reference for all purposes.
Organic electroluminescent devices (OLED) generally comprise an organic electroluminescent material sandwiched between two electrodes. Generally, the organic electroluminescent material is a multilayer structure comprising an electron transport layer, an electroluminescent layer and a hole transport layer. Upon application of an electrical current, the material radiates light generated by recombination of electrons and holes in the organic material. However, the organic luminescent materials are sensitive to contamination, oxidation and humidity. Further, in some OLED, the electrodes are also sensitive to contamination, oxidation and humidity. For example, Heeger teaches fabricating the OLED cathode electrode from a low work function metal such as calcium. However, Tang teaches that a cathode electrode made from reactive materials such as calcium are susceptible to oxidation. Tang teaches utilizing more stable materials with higher work function for the cathode electrode. The lack of an effective encapsulant to protect the OLED from the environment remains a major obstacle to OLED development.
Therefore, there is still a need in the art for an encapsulated organic electroluminescent device which is stable in an ambient environment.
It is therefore an object of the present invention to provide an improved encapsulated electroluminescent device.
Other objects and advantages will become apparent from the following disclosure,